1. Field of the Invention
This invention relates to the conversion of chemical energy to electrical energy. In particular, the present invention relates to a sandwich cathode for an electrochemical cell. The sandwich cathode comprises a first cathode active material mixed with non-active materials including conductive diluents and a binder to provide a first cathode active mixture having a relatively high rate capability in comparison to a second cathode active material mixture. However, the energy density of the first cathode active material mixture is greater than or equal to the second energy density of the second cathode active material mixture. The relative energy densities of the first and second cathode active material mixtures are tailored by the addition of the non-active conductive diluents and binder. In one embodiment, the first and second cathode active material mixtures are contacted to the opposite sides of an intermediate current collector. This cathode design is useful in electrochemical cells that power implantable medical devices requiring a high rate of discharge, stable operation, and predictable end-of-life.
2. Description of Related Art
Improvements in implantable cardiac defibrillators and the electrochemical cells that power them have enabled the use of a single cell to power a defibrillator. However, the requisite cell must have both a high overall energy density and a high rate capability. The capacity of the electrochemical cell is not only dependent on the electrode assembly design and packing efficiency, it also is dependent on the type of active materials used.
Certain patents have disclosed electrodes that provide a cell having both a high overall energy density and a high rate capability. For example, U.S. Pat. No. 5,744,258 to Bai et al. discloses a hybrid electrode for a high power, high energy, electrical storage device. The electrode contains both a high-energy electrode active material and a high-rate electrode active material. The first electrode active material has a higher energy density than the second electrode active material, and the second electrode active material has a higher rate capability than the first electrode active material. The two active materials are deposited on a current collector and the electrode is used to make an energy storage device that exhibits both the high-rate capability of a capacitor and the high energy capability of a battery. The materials can be co-deposited on the current collector in a variety of ways, either in superimposed layers, adjacent layers, intermixed with each other or one material coating the other to form a mixture that is then deposited on the current collector. The disclosure of this patent is incorporated herein by reference.
U.S. Pat. No. 6,551,747 to Gan, which is assigned to the assignee of the present invention and incorporated herein by reference, describes a sandwich cathode design having a first cathode active material of a relatively high energy density but of a relatively low rate capability sandwiched between two current collectors and with a second cathode active material having a relatively low energy density but of a relatively high rate capability in contact with the opposite sides of the current collectors. A preferred low energy density/high rate capability cathode active material is silver vanadium oxide (SVO), and a preferred high energy density/low rate capability active material is fluorinated carbon (CFx). The cathode design is useful for powering an implantable medical device requiring a high rate discharge application.
It is generally recognized that for lithium cells, silver vanadium oxide (SVO) and, in particular, ε-phase silver vanadium oxide (AgV2O5.5), is preferred as the cathode active material in high rate cell discharge applications. This active material has a theoretical volumetric capacity of 1.37 Ah/ml. By comparison, the theoretical volumetric capacity of CFx (x=1.1) is 2.42 Ah/ml, which is 1.77 times that of ε-phase silver vanadium oxide. However, for powering a cardiac defibrillator, SVO is preferred because it can deliver high current pulses or high energy within a short period of time. Although CFx has higher volumetric capacity, it is not useful as the sole cathode active material in medical devices requiring a high rate discharge application. This is due to its low to medium rate of discharge capability. That is one of the reasons the lithium-SVO/CFx cells disclosed by Gan in U.S. Pat. No. 6,551,747 are advantageous for providing both high energy capacity and high discharge rate in a single device.
Nonetheless, there remains a need for improvement in other aspects of cell performance. This includes the need for an electrochemical cell with the desired energy capacity and discharge rate, but with greater performance stability, reduced voltage delay, reduced cell impedance rise, and greater predictability at end-of-life discharge.